1. Field of the Invention
This invention relates to polyurethane golf ball covers which use an epoxy curing agent, in particular, (2,2-bis4-(2'3' epoxy propoxy) phenyl! propane). Golf ball covers employing the curing agents of the present invention have an improved shear resistance.
2. Description of the Prior Art
Polyurethane has been recognized as a useful material for golf ball covers since as early as about 1960. U.S. Pat. No. 3,147,324, filed Oct. 20, 1960, is directed to a method of making a golf ball having a polyurethane cover. This patent disclosed an improved method of covering a golf ball with a liquid urethane polymer which could be applied, set and cured at room temperature or at a temperature that would not damage the tensioned windings of the golf ball center. The curing agents disclosed were diamines, polyols or air moisture. However, the '324 patent does not teach the use of epoxy or diol curing agents and did not recognize the existence of shear resistance problems engendered in polyurethane golf balls which have been cured with diol curing agents.
Even though no commercial golf ball was introduced to the market until recently, various companies continued to investigate polyurethane as a golf ball cover material in limited ways. U.S. Pat. No. 4,123,061 issued Oct. 31, 1978 teaches that a golf ball can be made from a polyurethane prepolymer of polyether and a curing agent, such as a trifunctional polyol, a tetrafunctional polyol or a diamine. However, this patent does not recognize the fact that golf ball covers comprising polyurethane and a polyol curing agent exhibit reduced shear resistance. There is also no teaching in the '061 patent that the addition of an organic curing agent having at least one epoxy group to the polyurethane golf ball cover material will improve the product's properties in any respect.
Shear resistance is a golf ball's ability to withstand the shear force applied to a ball when hit with a golf club and/or iron. When the biting grooves on the striking surface of a golf club and/or iron impact a golf ball in a downward oblique swing causing it to be slidingly uplifted across, and immediately forcibly propelled in an outbound trajectory from, the particular club face, the shear force applied to the golf ball cover often produces cuts or abrasion marks on the surface of the cover material of the golf ball. Acushnet Company, which has been the industry leader in the research and development of polyurethane covered golf balls for twenty years, has only now unexpectedly found that polyurethane golf ball covers which incorporate an epoxy curing agent have significantly improved shear resistance.
Polyurethanes having an epoxy resin bound in their structure were known in the art, but not in the art of manufacturing golf ball covers. In particular, U.S. Pat. No. 4,870,142 to Czerwinski discloses polyurethane compositions which are produced from a polyisocyanate, a polyhydroxyl compound, a chain extender and an epoxy resin. The Czerwinski patent not only does not teach the use of polyurethane compositions in golf ball covers, but also does not disclose the problems of inadequate shear resistance that the present invention has now solved.
Despite the fact that it had been known for over thirty years that polyurethane compositions were useful in formulating golf ball covers, no commercially successful polyurethane covered golf ball appeared on the market until the introduction of Titleist's PROFESSIONAL brand golf ball in 1993. Although the cost of the raw materials needed to produce polyurethane composition golf ball covers was an inhibiting factor in bringing such golf balls on the market, the principal obstacle lay in the extraordinary difficulty in efficaciously applying a coating of polyurethane composition to a golf ball core to form a golf ball cover having a uniform thickness, i.e., a golf ball having a uniform difference between the inner and outer diameter of its cover.
In particular, the difficulty resided in centering a golf ball core in an amount of polyurethane that was sufficiently cured to keep the core centered while at the same time being insufficiently cured so that the cover material could be molded around the core. This problem was essentially solved and a commercially successful polyurethane covered golf ball appeared on the market with the introduction of the aforesaid PROFESSIONAL brand polyurethane covered golf ball in 1993. Ongoing research and development into different polyurethane compositions for golf ball covers has led to the present invention in which a golf ball core clad with a polyurethane composition cover which has been set with an epoxy curing agent is virtually free of abrading marks and damage to the cover when struck with a golf club.